De Haas was pitching for the Heurones in the Eerste Klasse when the Seattle Mariners signed him in January of 2008. After completing high school, he came to the US in June and began his career with the AZL Mariners. On June 22, he tossed a hitless inning against the AZL Padres, walking one, but things went south from there. After 8 1/3 IP, he had allowed 15 hits and 7 runs (6 earned) and opponents were hitting .405. He then took a break to join the Dutch junior national team for the 2008 World Junior Championship. He was 1-1 with a 8.10 ERA, taking a bad loss against South Korea and winning against Italy. He finished his first minor league season 0-1 with a 10.71 ERA and 1.91 WHIP.

In the 2009 European Junior Championship, Jeroen was 2-0 with a 1.59 ERA for the Netherlands, which finished second to Italy. He split the 2009 season between the AZL Mariners (1-0, 12.08, 20 H in 12 2/3 IP) and the Pulaski Mariners (11 H, 6 R in 6 2/3 IP).